American Environmental Fabrication & Supply uses VFD's on the main fan and any combustion fans used on the system. These are controlled by the operator interface, also commonly known as an Human Machine Interface (HMI) and provides a means for the Filtercrobe (bio-filtration system),regenerative, catalytic or direct fired direct thermal oxidizer operator to start and stop the motor and adjust the fans operating speeds. Additional operator control functions include switching between manual speed display and indication percentage of the fan speed or any faults on the VFD system. A separate VFD operator interface keypad and display unit is provided on the front of the VFD controller. The keypad and display are connected to the VFD controller. We provided integration from the VFD to the Human Interface device on the front of the control panel.

VFD OPERATION

When an induction motor is connected to a full voltage supply, it draws several times (up to about 3-6 times) its rated current. As the load accelerates, the available torque usually drops a little and then rises to a peak while the current remains very high until the motor approaches full speed.

By contrast, when a VFD starts a motor, it initially applies a low frequency and voltage to the motor. The starting frequency is typically 2 Hz or less. Thus starting at such a low frequency avoids the high inrush current that occurs when a motor is started by simply applying the utility or main power voltage at a switch. After the start of the VFD, the applied frequency and voltage are increased at a controlled rate or ramped up to accelerate the load without drawing excessive current. This starting method typically allows a motor to develop 150% of its rated torque while the VFD is drawing less than 50% of its rated current in the low speed range. A VFD can be adjusted to produce a steady 150% starting torque from standstill right up to full speed. However, cooling of the motor is usually not good in the low speed range. Running at lower speeds even with rated torque for long periods is not possible due to the overheating of the motor. If continuous operation with high torque is required in low speeds an external fan will be needed.

In principle, the current on the motor side is in direct proportion of the torque that is generated and the voltage on the motor is in direct proportion of the actual speed, while on the network or PLC side, the voltage is constant, thus the current on line side is in direct proportion of the power drawn by the motor, that is U.I or CAN where C is torque and N the speed of the motor.

With a VFD, the stopping sequence is just the opposite as the starting sequence. The frequency and voltage applied to the motor are ramped down at a controlled rate. When the frequency approaches zero, the motor is shut off. A small amount of braking torque is available to help decelerate the load a little faster than it would stop if the motor were simply switched off and allowed to coast. Additional braking torque can be obtained by adding a braking circuit (resistor controlled by a transistor) to dissipate the braking energy. With 4-quadrants rectifiers (active-front-end), the VFD is able to brake the load by applying a reverse torque and reverting the energy back to the network.